1. Field of the Invention
The present invention relates to a source of illumination or so-called feed horn for a telecommunications antenna.
2. Description of the Prior Art
The development of telecommunications, especially by satellite (reception and transmission of data, TV reception) calls for antennas which have excellent performance.
In order to ensure this overall performance, it is necessary to provide a source of illumination or feed horn which has symmetry of revolution and offers good polarization characteristics.
Nevertheless, the steadily increasing number of potential users dictates the need to design products which involve very low fabrication costs.
Conventional sources of illumination offering radiation patterns with good symmetry of revolution of the corrugated horn type suffer from the disadvantage of being very costly to manufacture.
Conventional trap-type feed horns offer the advantage or being more economical to construct but have low directivity which is nevertheless sufficient to illuminate a reflector of the parabolic type. These feed horns are composed of parts having an axis of symmetry (circular waveguide, trap, and so on) and must be fed by a standard rectangular waveguide. This feed is performed by means of a transition which may be either continuous or stepped.
Thus a French patent Application granted under No 2,096,684 describes a device for coupling a circular waveguide with a parabolic reflector of revolution which comprises a circular waveguide excited in the TE.sub.11 mode, a first end of this waveguide being located at the focus of said reflector and the other end being connected to a rectangular waveguide. An incomplete cylindrical annular cavity surrounds the first end of the waveguide. A transition element formed by a series of waveguide sections having progressively varying shapes and dimensions establishes a coupling with the rectangular waveguide.
However, a continuous transition must be of appreciable length in order to obtain good matching characteristics. A stepped transition as described in the patent Application cited earlier permits a reduction in length but is difficult to fabricate in accordance with low-cost technologies.
The object of the present invention is to overcome these disadvantages.